PREVENTION OF BREEDING 459 



ten days. Films of heavier oils or heavy and light oils mixed 

 last longer, and need be renewed only once in two, three or four 

 weeks, according to conditions. Thin oil will spread into a film 

 if simply poured on the surface, but heavier oils are best sprayed 

 on. In Africa mops made by tying kerosene-soaked cloths on 

 long sticks are used for spreading the oil and in Panama waste 

 cloth soaked in oil is placed where a slow flowing stream will 

 constantly take a thin film from it. 



In the tropics the use of petroleum has often been found im- 

 practicable on account of the rapid evaporation, continued 

 heavy rains, and the interference made by the luxuriant and 

 rapid growth of water plants and algae and the formation of an 

 interfering scum from a combination of the oil and dead algae. 

 For this reason substances which are actively poisonous to the 

 larvae and which form an emulsion in the water are used instead. 

 An almost ideal larvicide of this type is now made at Ancon, 

 C. Z., in enormous quantities. It is made of crude carbolic acid, 

 powdered resin and caustic soda, heated together to make a black 

 liquid resin soap which readily forms a milky emulsion with 

 water. It destroys Anopheles larvae in 16 minutes in an emul- 

 sion of one part in 5000. It also kills larvae in mud, and destroys 

 grass, algae and water weeds in which larvae ordinarily hide. 

 In making the substance 150 gallons of crude carbolic acid are 

 heated to 100 C., 150 to 200 Ibs. of powdered resin stirred in, 

 30 Ibs. caustic soda added and the whole stirred and kept hot 

 until the black liquid soap is formed. 



Prevention of Breeding, and Natural Enemies. The most 

 valuable method of reducing mosquitoes, where practicable, is 

 to obliterate breeding places or to make them uninhabitable 

 for the larvae. The first step in reducing mosquitoes is to see 

 that there are no flower-vases or other water receptacles serving 

 as aquaria for the larvae, that there are no water-filled tin cans 

 in the garbage heap or that the roof or street gutters do not 

 hold standing water. Any rain barrels, cisterns, cesspools or 

 small reservoirs which cannot be disposed of can be made harm- 

 less by screening. Pieces of low ground, temporary pools, etc., 

 can usually be eliminated by draining. 



The natural enemies of mosquito larvae can often be exploited 

 successfully for destroying them. Dr. Smith found that one of 

 the most potent factors in the reduction of mosquitoes in the 



